An insulated-gate bipolar transistor (IGBT), which is a semiconductor power switching element, is used in many power switching control devices for electric systems. For instance, it is used in an ignition device for internal combustion engines as disclosed in U.S. Pat. No. 5,664,550 (JP 3,216,972). In this ignition device, the IGBT is turned on and off by a control signal from an electronic control unit for an engine (engine ECU), so that current supply to the primary winding of an ignition coil connected to the IGBT is controlled to control spark discharge of a spark plug. Specifically, the IGBT is turned on and off when the control signal becomes high and low, respectively.
If the control signal from the engine ECU continues to be high too long due to some abnormality, the IGBT continues to be turned on for a long time in and causes overheating. It is therefore necessary to detect the excessive continuation of current supply and prohibit the IGBT from continuing to be turned on, so that the IGBT is protected from breaking down due to overheating.
Various methods are proposed to detect the excessive continuation of current supply. One method is to monitor the control signal. The excessive continuation may be determined when the control signal continues to be high for more than a predetermined time period. Another method is to detect the temperature of the IGBT. The excessive continuation is determined when the IGBT temperature rises to a predetermined temperature.
According to the latter method, the IGBT temperature may be detected based on the temperature characteristic of diodes formed in a switching integrated circuit (IC), in which the IGBT is formed. That is, this method is based on the forward voltage drop of diodes varies with temperature. For instance, an overheat detector circuit is formed with at least one diode and current is supplied to the overheat detector circuit from a current regulator circuit. The excessive continuation of current supply is determined when a terminal voltage of the overheat detector circuit falls below a predetermined voltage. With the detection of IGBT temperature, the IGBT is protected from overheating, which may be caused by not only the excessive continuation of current supply but also other causes. Therefore, it is preferred to determine the excessive continuation based on the IGBT temperature.
If the IGBT is formed as a vertical type power element, electrodes need be connected to both top and bottom faces of the switching IC to detect the IGBT temperature. For instance, the electrodes may be placed on a frame on which the output terminal of the switching IC is formed so that the bottom face of the switching IC may be connected to the frame. A monitor circuit for monitoring the terminal voltage of the overheat detector circuit may be formed in a control circuit IC, which is distanced from a frame forming a current path to the IGBT. That is, the monitor circuit is formed in the control circuit IC, which is formed as a chip different from the switching IC including the IGBT.
When the switching IC and the control circuit IC are formed separately, these ICs must be connected with wires so that the overheat detector circuit and the terminal voltage monitor circuit. This construction is likely to be affected by electromagnetic noise, because the wires have inductive components (L) and capacitive components (c) are formed between the wires and the frame, etc. The electromagnetic noise causes LC resonance with the L components and the C components, thus changing the diode forward voltage drop. As a result, the protective function against overheating is initiated by the overheat detector circuit even when the IGBT temperature is within a normal temperature range. In this case, spark discharging is disabled notwithstanding normal IGBT temperatures. Not only the ignition device for engines but also other electric systems, in which electric power supply is switching-controlled, suffer from the same drawback.